This invention relates to facsimile equipment and more particularly to the generation of frequency modulated facsimile signals representing dark/light variations or light/dark variations on a document.
Facsimile transmitters or transceivers having a transmitting capability frequency generate frequency modulated signals which are substantially sinusoidal in waveform. Typically, these sinusoidal signals vary in frequency from 1500 Hz. representing white to 2400 Hz. representing black with various intermediate frequencies representing shades of gray. It is common practice to generate the sinusoidal signals of utilizing a voltage controlled oscillator which produces an output of the desired frequency in response to voltages representing the dark/light variations as detected by a light sensitive device. The output from the voltage controlled oscillator which has a frequency equal to the frequency of the facsimile transmission signal is then filtered to remove high frequency components. This filtering is necessary to eliminate the high frequency components but undesirably decreases the speed of response of the transmitter to instantaneous dark/light variations so as to introduce phase distortion and delay. U.S. Pat. Nos. 3,911,207 and 4,015,077 assigned to the assignee of this invention, disclose facsimile equipment which generates facsimile signals in the above-described manner.
In general, it is desirable to transmit the dark/light transmission information over appropriate communication links, e.g., telephone lines, as rapidly as possible so as to maximize the use of the available bandwidth and provide optimum document resolution. This requires that the transmitting circuitry be capable of rapid changes in frequency representing a dark/light variation or light/dark variation. In other words, it is desirable that the frequency modulated facsimile signal be capable of a substantially instantaneous change in slope.
In the prior art, the facsimile signals have not been capable of the desired instantaneous change in frequency. Rather, the prior art has been characterized by a limited speed of response which can result in output signal phase distortion. Where the facsimile signal has been coupled to the network acoustically, the delays have not been that critical since acoustic coupling effectively limited the faithful reproduction of dark/light variations even if high frequency shift rates were achievable. However, with the advent of liberalized direct coupling for facsimile transceivers as disclosed in copending application Ser. No. 689,263 filed May 24, 1976, it becomes more important to assure that the speed of response is limited only by the unpredictable characteristics of the telephone network itself and not the facsimile transmission equipment.
The prior art includes techniques for synthesizing sinusoidal waveforms using digital techniques. In this connection, reference is made to an article entitled, "Create Sinewaves Using Digitalized IC's," Radio Electronics, November, 1976. As shown there, the output from the counters are summed to achieve a step-like sinusoidal waveform which is then smoothed by filtering.